Lighting devices based on optical fibres are well known in the prior art. These optical fibres generally consist of a core and an outer sheath surrounding the core.
Document U.S. Pat. No. 8,215,810 describes lighting devices constituted by a translucent casing and a plurality of optical fibres based on poly(methyl methacrylate) (PMMA) inserted into the casing to hold them in place. The optical fibres undergo a prior homogeneous treatment of their sheath in order to create surface modifications which expose their core. Thus, by connecting the fibres to a light source, some light propagates in the fibres and emerges through the surface modifications created in their sheaths. However, these devices have the disadvantage of not diffusing the light homogeneously along their entire length. Indeed, devices of too great a length notably show light attenuation as the light source gets further away. In this case, the intensity of light diffused within the device therefore decreases as a function of the distance relative to the light source.
To overcome this disadvantage, this document proposes an alternative consisting of placing a light source at each end of the lighting device. However, this alternative has limitations, since for long lengths, the problem remains identical. Furthermore, besides the cost and the complexity of the assembly, the light sources are generally not exactly alike, more particularly in terms of colour and intensity. A contrast in intensity and/or colour is therefore generated and visible in the lighting device. Thus, there is no known manufacturing method for the reliable production of this type of lighting device.
As taught by document U.S. Pat. No. 6,450,678, a lighting device comprising a casing including optical fibres based on PMMA having undergone a surface treatment to create surface modifications is also known. As described in this document, this device is used for light-marking the periphery of an object, for example a car seat. Once mounted, the device can therefore have highly curved areas, forming acute angles. So for the same density of surface modifications created on a sheath of an optical fibre, light escapes more significantly from an acute angle than from a straight section. The device, once mounted, therefore has areas of greater brightness that impact the quality of the desired marking. According to this document, the fibres are treated so as to avoid the appearance of such areas. The treatment of the sheaths of the fibres is thus adapted to the curvature that the lighting device undergoes when it is mounted on its support, typically a vehicle interior article. Notably, the number of surface modifications carried out in the sheaths of the fibres is decreased in the areas of fibres having greater curvature. Thus, the device is dedicated to a very specific application and requires either a manufacturing method specific to it, or the performance of additional or related steps, which are restrictive. This represents a significant disadvantage for the manufacturing of lighting devices on a large scale.
The method of forming surface modifications on the sheaths of optical fibres is known to a person skilled in the art. A known technique, for example described in document FR 2 714 147, consists of scrolling free optical fibres in a blasting cabinet where a nozzle spraying an abrasive under high pressure is positioned. To reliably obtain the expected result, it is therefore necessary to ensure controlled scrolling of the fibres in the cabinet, and to prevent them from bending during treatment under the effect of the high pressure exerted. This therefore assumes the use of a scrolling mechanism that pulls the fibres sufficiently to hold them in place during blasting, hence a risk of fibre breakage.